This invention relates to a photo sensor with bias regulator to keep a photo diode reverse voltage low and relatively invariant with supply voltage changes and more particularly relates to such a regulator that is controlled by an auxiliary photo diode.
Photo diode voltage regulators are described by R. Genesi in his U.S. Pat. No. 4,085,411, issued Apr. 18, 1978, and by W. Gontowski, Jr. in his U.S. Pat. No. 4,199,753 issued Apr. 22, 1980. Another is described by W. Gontowski, Jr. and E. Chalfin in the above noted parent patent application. Those two patents and that application are assigned to the same assignee as is the present application. By means of a photo-diode-current feedback circuit, the Genesi circuit holds the reverse voltage across the photo diode near zero for a wide range of light intensities and thus a wide range of photo-diode currents so that sharp changes in light intensity are faithfully followed without delay in the photocurrent. The Gontowski regulator consists in a modified current mirror type current source that electrifies a V.sub.BE -multiplier circuit providing a log-log noise transfer function from the DC supply to the photo diode. Both of these circuits inherently draw a DC component of current from the DC power supply.
A photo sensor including a photo diode and a current mirror amplifier is described in the parent patent application. There the photo diode is connected between the input of the first current mirror stage to the input of a subsequent stage, both stages being formed of one polarity-type transistors; e.g. NPN. A low photo-diode reverse voltage may be achieved in this way, which voltage is also substantially invariant with changes in supply voltage. This simple photo diode biasing method furthermore does not draw any standby current when there is no ambient light and does not draw a DC component of current from the power supply. It may, however, be subject to oscillations or may not start under the conditions that the DC power is applied slowly rather than being switched on.
It is therefore an object of this invention to provide a stable photo sensor that requires no steady start current, draws no other DC component from the DC power supply and starts reliably however the supply voltage is applied.
It is further object of this invention to provide such a linear two-terminal type photo sensor that draws from the DC supply a current that is directly proportional to the ambient light intensity.